1. Field of the Invention
The invention is generally related to a lift boat or jack-up rig and more particularly to the mechanism for raising and lowering the legs of a lift boat or jack-up rig.
2. General Background
In offshore work related to the search for and production of oil and gas, a variety of vessel types are used. One type is a lift boat. A lift boat is a vessel that can elevate itself out of the water so as to provide a stable platform at the appropriate elevation to perform a number of marine construction tasks. Lift boats are equipped with retractable legs that each has a footing at the bottom. The footings contact the bottom and are of sufficient size to support the vessel on the seabed. The number of legs can vary from three to as many as six. One or more cranes are fixed to the deck of the vessel and are used to lift equipment onto or off of oil drilling or production platforms. A larger version of the lift boat called a jack-up rig typically is outfitted with drilling equipment. From this point on all mention of lift boats shall also be understood as including jack-up rigs.
At least one gear rack is typically incorporated into each leg of a lift boat. The legs of a lift boat are either constructed as a lattice type or as a tubular type. One or more pinion assemblies operate along each gear rack. A pinion assembly typically consists of a pinion, gear box, braking mechanism and either an electric or hydraulic motor. The pinion assemblies are either rigidly fixed to the vessel or can be of the floating type. As the pinions of the lift boat rotate, the lift boat is either raised out of the water or lowered toward the surface of the water depending upon the direction of pinion rotation.
The legs can be somewhat self-centering if multiple gear racks are used on the legs and if the gear racks are arranged properly. Even if the racks are ideally numbered and positioned some side loading of the legs will occur due to sea, wind, and vessel loading conditions. The current generation of lift boats employs a linear metal bearing guide to restrict leg movement. This guide system consists of metal bearing strips attached to the vessel or to the jacking apparatus. The guides may ride along the gear rack, the leg cords, or attachments to either the leg or gear rack. Smaller lift boats have leg towers constructed from tubular members and have tubular legs with outside diameters slightly smaller than the inside diameters of the leg towers. The leg tower is the sole guide. The shortcomings of these types of guide apparatus are that friction between the leg and guides increases the jacking force required to operate the lift boat and much of the lubricant used on the guides is dropped into the sea.
The present invention addresses the above needs in a straightforward manner. What is provided is an apparatus for efficiently guiding the legs of a lift boat. Roller assemblies are used to guide the legs. The rollers may be placed at any location or in any number either vertically or around the leg to adequately center the leg. The roller can either have a metal surface that rolls along the leg or be coated with a resilient material. The base of the roller can either be rigidly mounted to the vessel or incorporate resilient material between the roller and the vessel. A means of adjusting the clearance between the leg and roller may be incorporated in the roller assembly.